In April 1863, President Lincoln officially announced West Virginia would join the Union on June 20, 1863. Berkeley and Jefferson counties were added later when they were under Union military control.

West Virginia's borders were mostly based on existing county lines, creating a new border with Virginia. Although people often talked about the mountains separating the regions, the final border didn't always follow mountain ranges. This split was the only permanent change to a state's boundary caused by the Civil War.